Traditionally the flight control surfaces of commercial airliners are hinged about rolling element bearings. These bearings provide a low co-efficient of friction, a low and largely predictable wear rate, and in some instances a misalignment capability that allows for a small angular mismatch between the flight control and the wing attachment.
Such bearings can be considerably larger in diameter than the attachment pin that joins the flight control surface to the wing. The large diameter of the bearing results in a considerably larger hole in the flight control or wing structure than the attachment pin necessitates. This large hole in the structure often has a significant weight penalty as a result of the fatigue performance associated with the interruption in stresses flowing around the hole.
A further problem with the large diameter of such bearings is that they make it difficult to position the hinge line close to the outer surface of the wing without creating a protrusion which would require aerodynamic shrouding. Positioning the hinge line close to the outer surface of the wing is desirable because it maximises the distance between the hinge line and the line of action of the actuator jack which rotates the control surface. This enables the size and weight of the actuator to be minimised, reducing or even eliminating the need for fairings to cover the actuator should its size result in portions of its body being unshrouded by the wing.
A plain bearing, or friction bearing, works by operation of a low friction slip path between two parts, enabling them to move relative to each other without requiring any rolling elements (such as ball-bearings) between them. A plain journal bearing is a type of plain bearing which enables the parts to rotate relative to each other. Plain journal bearings have not traditionally been used for flight control surfaces since they do not have the necessary redundancy. In other words, if the low friction slip path becomes jammed, then there is no secondary slip path which will enable the bearing to continue to operate.